“Matt Dickinson”:http://blogs.middlebury.edu/presidentialpower/2011/01/07/why-gibbs-really-resigned-and-why-daley-was-hired/:
bq. The bottom line? The modern White House staff moves to a somewhat predictable rhythm, one dictated by a president’s administrative needs. When a president moves from governing to campaign mode, as Obama is now doing, the expertise he seeks from his senior aides changes as well. This invariably produces higher level of staff turnover. News accounts often portray these changes in terms of individual factors, such as staff burnout, personality clashes, the aide’s perceived ineffectiveness or an internal power struggle (Daley forces Gibbs Out!) While not totally discounting these factors, the primary cause for the staff overhaul that we see in the Obama presidency now is systemic, not individual. Simply put, Obama is running for reelection and he is reshaping his White House accordingly.
Some nice data in his post as well.